Optical pattern tracers and in particular those of the circular scanning non-steering type are disclosed for example, in U.S. Pat. No. 3,704,372 issued Nov. 28, 1972 to Robert E. Parker et al; U.S. Pat. No. 3,727,120 issued Apr. 10, 1973 to George S. Jewell et al; U.S. Pat. No. 3,860,862 issued Jan. 14, 1975 to William Dell et al and U.S. Pat. No. 3,883,735 issued May 15, 1975 to Francis P. Murphy et al. These tracers normally scan the pattern by reflecting a portion of the pattern from a mirror onto an optically sensitive device. The mirror is rotated to cause the portion of the pattern viewed to rotate about a center and produce a circular scan. The signal produced by the optically sensitive device may then be processed and produce coordinate velocity signals which may be used to cause the tracer to follow in the pattern at a constant tangential velocity. All pattern followers must view the pattern at a point in advance of the actual axis of steering or physical rotation of the apparatus. Failure to provide sufficient advance will result in system instability or inability of the tracer to follow rapid deviation. The degree of advance is related to the accuracy of tracing and a trade-off occurs between the stability of the system, the accuracy of tracing and the velocity of tracing. Various systems have been proposed in the past to enable a tracer to operate at relatively high tracing speed and slowdown in advance of rapid deviations thus enabling accuracy at the turns and high velocity between turns. Such a system is disclosed in U.S. application No. 626,221 filed June 29, 1984 in the name of Robert E. Parker, et al. In that system two scans of different diameter are used. One scan is used to detect rapid deviation well in advance of the tracer and the other scan is used for normal steering purposes.
The prior art for this invention is disclosed in the foregoing Application where coincidence of the pulse from the normal scanner and from the advance scanner is used to determine whether or not the machine should slow down. The absence of coincidence or overlap between the pulses from both scanners, for a period of more than one scanning cycle results in a slowdown signal.